Meiosis Tutorial

Meiosis I & II

In meiosis I, chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step in meiosis that generates genetic diversity.

The phases of meiosis I & II

Prophase I

DNA replication precedes the start of meiosis I. During prophase I, homologous chromosomes pair
and form synapses, a step unique to meiosis. The paired chromosomes are called bivalents, and the formation
of chiasmata caused by genetic recombination becomes apparent. Chromosomal condensation allows these to be
viewed in the microscope. Note that the bivalent has two chromosomes and four chromatids, with one chromosome coming
from each parent.

Prometaphase I

The nuclear membrane disappears. One kinetochore forms per chromosome rather
than one per chromatid, and the chromosomes attached to spindle fibers begin to move.

Metaphase I

Bivalents, each composed of two chromosomes (four chromatids) align at the metaphase plate. The orientation
is random, with either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells
to get either the mother's or father's homologue for each chromosome.

Anaphase I

Chiasmata separate. Chromosomes, each with two chromatids, move to separate poles.
Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids.

Telophase I

Nuclear envelopes may reform, or the cell may quickly start meiosis II.

Meiosis II is similar to mitosis. However, there is no "S" phase. The chromatids of each chromosome are no longer
identical because of recombination. Meiosis II separates the chromatids producing two daughter cells each
with 23 chromosomes (haploid), and each chromosome has only one chromatid.